Suspension polymerization is a heterogeneous radical polymerization process that uses mechanical agitation to mix a monomer or mixture of monomers in a liquid phase, such as water, while the monomers polymerize, forming spheres of polymer. For many non-polar monomers, polymerization in an aqueous dispersion offers a method of eliminating many of the problems in bulk and in solution polymerization, especially the heat dissipation problem in the former and solvent reactivity and removal in the latter. Another feature for large batch preparations is that the polymeric products obtained from a suspension polymerization, if correctly carried out, are in the form of finely granulated beads, typically 0.05 to 1 mm in size. Such beads are readily filtered and dried, and then are readily stored, transported, and directly used by the manufacturing processes used to make final articles or products from the suspension polymerized polymers. The aqueous dispersion polymerization method known as emulsion polymerization utilizes water soluble initiators and forms much smaller polymeric particles, which do not have the ease of handling properties that suspension polymerized beads confer. Polymer prepared in this way is in the form of an extremely fine powder, generally less than 1 micron in size, which must be consolidated by melting, compressing or tabletting to bring it into a form suitable for final use.
Rates and molecular weights for a suspension polymerization reaction are identical to those expected for bulk polymerizations. The catalyst is dissolved in the monomer, the monomer is dispersed in water, and a dispersion agent is incorporated to stabilize the suspension formed. The monomer can include a mixture of different monomers, such that the polymeric product is a copolymer. The catalyst or polymerization initiator should be soluble in the monomer, but not in water. The suspending or dispersion agents which are generally used fall into one of two classes, water soluble organic polymers or water insoluble inorganic compounds. Examples of the former are poly(vinyl alcohol), poly(acrylic acid), methyl cellulose, gelatin, and various pectins. Examples of the latter are kaolin, magnesium silicates, aluminum hydroxide, silica, calcium carbonate, and calcium phosphate. The suspending agents are believed to stabilize the suspension primarily by preventing or reducing the number of direct collisions between droplets. The inorganic compounds are, in general, more readily removed from the final polymer than are the organic polymer agents. The useful range of the weight ratio of the organic phase monomers to the aqueous phase is typically higher for the inorganic stabilizers than the water soluble organic polymers, thus providing an economic advantage. Maintenance of colloidal stability of the suspension during the course of the polymerization process, typically a number of hours, is critical to industrial practice of the method.
Useful monomers for suspension polymerization include, but are not limited to, styrene, vinyl toluene, o, p or m chlorostyrene, chloromethyl styrene, divinyl benzene, ethylene glycol dimethacrylate, vinyl chloride, vinylidene chloride, vinyl acetate, alpha olefins, butadiene, isobutylene, acrylic monomers such as methyl acrylate, methyl methacrlyate, butyl acrylate, and butyl methacrylate. Monomers having significant water solubility, such as acrylic acid or acrylonitrile, can be polymerized efficiently with a suspension process by adding electrolytes to salt out monomer from aqueous solution.
Monomer soluble polymerization initiators and catalysts include, but are not limited to, various peroxides and azo compounds. These materials are generators of free radicals which initiate the addition polymerization of unsaturated monomers. Useful materials are commercially available with differing half-lives at differing temperatures; those skilled in the art can use variables such as concentrations, polymerization temperatures, and choices of initiators or blends thereof to control the rates of polymerization and resulting molecular weights and molecular weight distributions of the final product polymers. Examples of peroxide initiators include benzoyl peroxide, tert-butylperoxide, diacetyl peroxide and lauroyl peroxide. Examples of azo initiators include 2,2′-azodi(isobutyronitrile) and 2,2′-azodi(2-methylbutyronitrile).
Practical problems encountered in suspension polymerization due to inadequate colloidal stability include unstable monomer droplet size, the formation of coagulum that is difficult to wash and separate from the beaded polymer product, and the fouling of the reactor and stirrer surfaces.
U.S. Pat. No. 2,932,629 to Wiley describes suspension polymerization of monomers including styrene, divinyl benzene, chloromethylstyrene and vinylidene chloride using particulate stabilizers including inorganic particles such as bentonite clay and other hydrous mineral oxides, as well as organic particles including raw starch and sulfonated cross-linked polystyrene resins. The use of particles as stabilizing agents is shown to result in desirable uniform size distribution of the resultant polymer compared to suspension polymerizations conducted with a water soluble polymer stabilizer. Wiley suggests the use of low concentrations of water soluble promoter materials such the equimolar condensation product of diethanol amine and adipic acid to assist in the adherence of the particulate stabilizer materials to the interface between the discontinuous organic monomer phase droplets and the continuous aqueous phase. Such a promoter material is useful in the practice of the current invention. Wiley also describes the use of cupric sulfate pentahydrate to inhibit polymerization of monomers in the aqueous phase, which can cause destabilization of the suspension. The cupric sulfate pentahydrate also functions to cause gelation of a thickener such as additional bentonite clay dispersion added after the initial shearing to form monomer droplets in the presence of the particulate stabilizer. It is not desirable to use soluble copper salts in a commercially practical suspension polymerization process due to the adverse environmental effects of copper ions in the rinse water used at the end of the process.
U.S. Pat. No. 2,566,567 to Hutchinson et al. discusses the production of polystyrene beads by a suspension polymerization process. Hutchinson et al describe the difficulty of maintaining the size of beads and control of the process. They assert that “styrene and styrene derivatives pass, during the course of their polymerization, through a “tacky” or “sticky” stage when the tendency of particles thereof to coalesce is very pronounced and a subsequent disruption or separation of coalesced particles is difficult. They propose a two-stage process where a partially polymerized viscous liquid mixture of a monomer and a polymer is dispersed in a heated aqueous medium until the polymerization is completed. They also propose a complex vertical reaction system to conduct such polymerizations.
Commonly-assigned U.S. Pat. No. 4,912,009 to Amering et al. describes the production of toner which uses a suspension polymerized styrene-acrylic copolymer as the binder resin. The suspension polymerization process incorporates a complex of hydrophilic silica with a polyester promoter polymer of a lower alkylene dicarboxylic acid and an amino alkanol as the stabilizing species. An example of the promoter is poly(2-methylaminoethanoladipate). Amering et al state that a water soluble substance is required to prevent the emulsion or solution polymerization of monomers in the aqueous phase, a preferred example of such is potassium dichromate. Others are said to include sodium nitrite, copper salts and phenols, however the only example given is potassium dichromate.
U.S. Pat. No. 7,445,879 to Hsieh et al describes the direct preparation of toner by a suspension polymerization method. Ingredients necessary to the function of a suspension polymerized polymeric bead as a toner such as pigment, wax and charge control agent are added to a styrene, butyl methacrylate, azo free radical initiator mixture. This organic phase is dispersed in an aqueous phase where silica is the suspension stabilizer, polyvinylamine is the promoter species, and sodium dichromate is the water soluble free radical scavenger to enhance suspension stability. They also discuss various optional surfactants including water soluble polymers.
U.S. Pat. No. 5,414,062 to Lundquist describes a process for reducing the formation of aqueous phase polymer during the suspension polymerization of vinyl monomers. The reduction of polymer formation in the aqueous phase and subsequent fouling of polymerization reactor surfaces is achieved by incorporating an effective amount of water-soluble peroxide compound into the aqueous phase used for the suspension polymerization. Hydrogen peroxide, sodium perborate and sodium percarbonate are effective aqueous phase inhibitors for use in the preparation of cross-linked polystyrene beads. The suspension stabilizer used in the examples of U.S. Pat. No. 5,414,062 is a mixture of sodium hydroxide, boric acid, polyacrylic acid polymer and gelatin. The reaction is run at a disadvantageous low level of monomers of about 10%.
U.S. Pat. No. 3,995,096 to Flatau et al. describes the reduction of encrustations on reactor walls during the suspension polymerization of vinyl chloride when hydrogen peroxide is used in the aqueous phase at 0.0005% to 0.05% based on the weight of monomers. The patent states that other oxidizing additives including permanganates, dichromates, chromates, cerium(IV) sulfates, copper oxide, and lead oxide among others are less effective in reducing reactor fouling.
Commonly-assigned U.S. Pat. No. 9,029,431 to Nair et al. describes the preparation of porous polymer particles by a suspension process where an organic phase comprising solvent including dissolved resin and dispersed hydrocolloid is dispersed in an aqueous phase containing a particulate suspension stabilizer plus other co-stabilizing species. Porous particles are produced upon removal of the solvent, the shape of which is controlled by use of a water soluble shape control agent added to the aqueous phase after the dispersion of the organic phase in the aqueous phase but before the removal of the solvent. Without the shape control agent the particles are spherical; with shape control agent irregular and folded shapes can be produced. Polyethyloxazoline is listed among other species as a more desirable shape control agent.